Current methods of growing single crystal ingots in a Czochralski-type crystal-growing furnace typically involve melting a polycrystalline host material, such as silicon, and a measured amount of dopant together in a crucible to create a melt. Once the melt is prepared, a seed crystal is lowered into contact with the melt to begin the crystal-growing process. As the seed crystal is slowly extracted from the melt, a monocrystalline crystal or ingot is drawn from the melt. Unfortunately, the monocrystalline ingot does not necessarily include a proportionate share of the dopant in the melt. Instead, the percentage of dopant incorporated into the monocrystalline ingot depends on the applicable segregation coefficient and other parameters.
Typically, the ingot incorporates a smaller percentage of the dopant than the melt. As such, the dopant concentration in the melt will increase over the crystal-growing period as the ingot is drawn from the melt. Due to the increasing dopant concentration in the melt, the ingot will also gradually incorporate a larger amount of dopant as the growth process proceeds. Since the resistivity of the ingot is a function of the amount of incorporated dopant, ingots that incorporate a increasing amount of dopant over their length also have a resistivity that decreases over their length. As a result, the wafers into which an ingot is sliced will also have slightly different resistivities depending upon the relative lengthwise location from which each wafer was sliced. Since purchasers of the wafers typically specify an acceptable range of resistivity values depending upon the intended use of the wafer, only a subset of the wafers harvested from an ingot may satisfy the requirements imposed by a purchaser.
U.S. Pat. No. 5,406,905 to Yemane-Berhane et al. discloses a technique for doping the melt after the host material has been melted in the crystal-growing furnace. This technique involves casting the dopant around the seed crystal used to grow the ingot. When the furnace is prepared, the dopant-coated seed crystal is held in a relatively cool part of the furnace until the host material has melted and is ready for doping. The dopant-coated seed crystal is then lowered to a position just above the melt. Heat transferred from the melt to the dopant-coated seed crystal causes the dopant, in solid form, to slip off the seed crystal and into the melt, hopefully without splashing and without immersing the seed crystal in the melt.
However, the '905 Yemane-Berhane et al. patent does not address the problems associated with variations in the concentration of the dopant throughout the course of drawing an ingot from the melt. Instead, when the temperature of the seed crystal rises due to the heat from the melt, a point is attained where the dopant will slide off the seed. Here, all of the dopant is delivered to the melt before the seed crystal is immersed in the melt to begin the ingot growing process. Thus, this technique is still prone to the problem of the conventional techniques wherein, as the crystal is grown, the concentration of the dopant in the melt will continually change, thereby altering the resistivity profile of the ingot in a lengthwise direction.
U.S. Pat. No. 5,242,531 to Klingshirn et al. discloses a process for continuously recharging a melt crucible with additional molten host material and additional molten dopant. In this regard, the Klingshirn '531 et al. patent describes separate containers filled with the host material and the dopant that are positioned above the melt crucible. Feedlines connect the containers with an additional crucible or container in which the host material and the dopant are mixed and melted. This additional crucible includes an outlet for supplying additional molten semiconductor material to the melt in order to recharge the melt during the crystal-growing process. While the '531 Klingshirn et al. patent addresses some of the issues with respect to controlling the amount of dopant in the melt throughout the course of a crystal-growing process, the technique described by the '531 Kingshirn et al. patent requires multiple containers positioned above the melt crucible which may complicate the design of the crystal-growing furnace and limit access to the melt crucible during the crystal-growing process.
Therefore, a need still exists for improved techniques of controlling the amount of dopant in the melt throughout the course of the crystal-growing process without requiring significant modifications to the crystal-growing furnace and without incurring the attendant costs. Consequently, a need still exists for improved techniques for controlling the concentration of dopant incorporated into the ingot over the length of the ingot, thereby also permitting the resistivity of the resulting wafers to be controlled.